Recent research on English-medium instruction (EMI) has examined university instructors' professional identities in science and social science rather than in the humanities. To fill the gap, this qualitative case study explores the ways in which 13 university instructors in law and the humanities in Taiwan perceived their identity and exercised their agency as teachers. The findings indicated that these instructors had three ideal teacher identities, including educators in global and local contexts, subject matter instructors, and EMI instructors. Each determined teachers' choice, design, and enactment of EMI practices, as well as their notions of language use and objectives. This study highlights the primacy of preferred teacher identities in the manifestation of teacher agency, identifies enabling and constraining effects of teacher agency, and emphasizes the significant roles of discipline-specific communities and ethnicity and nationality in EMI practices. As such, teacher identities should be taken into consideration in professional development to empower instructors to take actions that maintain their commitments to EMI.